The present invention relates in general to FM demodulators, and more particularly, to an FM demodulator with a self-tuning quadrature detector.
FM demodulators are commonly used in television receivers where a transmitted signal must be demodulated to extract the audio and video signals from the RF (radio frequency) carrier. In a simplified view, the video information is AM modulated on a video carrier f.sub.c and the audio information is FM modulated on a sound carrier f.sub.s. The transmitted RF signal frequency is reduced through a tuner to an IF (intermediate frequency) signal and demodulated to baseband. After demodulation of the IF signal, the video information becomes a video baseband signal, while the audio information remains modulated on a sound intercarrier (f.sub.c -f.sub.s) at the difference between the video carrier f.sub.c and sound carrier f.sub.s. In the United States, the video carrier f.sub.c is set to 45.75 MHz, and the sound carrier f.sub.s is set to 41.25 MHz, leaving the f.sub.c -f.sub.s sound intercarrier at 4.5 MHz. The demodulated IF signal may be further processed through a trap (band-reject) filter to remove the f.sub.s sound carrier and retrieve the video information as luma (black and white) and chroma (color). An RGB signal may be generated from luma and chroma to drive the CRT screen. Likewise, the demodulated IF signal may be processed through a band-pass filter to isolate the sound intercarrier and then FM demodulated with a quadrature detector to retrieve the audio information.
One difficulty for FM demodulators in television receivers is the variety of carrier frequencies used worldwide for television signal transmission. There does not exist a single, worldwide television carrier frequency standard. In the audio processing path, the quadrature detector typically uses a fixed inductor-capacitor (LC) tank circuit tuned to produce a 90.degree. phase shift between the inputs of a multiplier at a predetermined sound intercarrier frequency. Unfortunately, the sound intercarrier frequency may vary between say 4.5 MHz and 6.5 MHz depending on the country of operation. Therefore, the quadrature detector manufacturer must be aware of the importing country's standard in order to properly pre-tune the LC tank circuit. This customization adds costs to the commercial manufacturing and complicates the distribution process.
With the multi-frequency standards, a need exists for a quadrature detector capable of self-tuning over a range of frequencies in a well determined manner while improving linearity.